For forming containers, films, sheets and the like, melt molding is often employed, in general. Therefore, resin compositions for use in forming these containers and the like are desired to be capable of forming containers and the like that are superior in appearance characteristics without occurrences of defects such as fish eyes and streaks, and to be superior in long-run workability not accompanied by occurrences of the aforementioned defects even if melt molding is carried out for a long time period, and the like. In addition, the fish eyes and the like not only deteriorate the appearance of containers and the like, but also cause deterioration of performances; therefore, a composition capable of suppressing these occurrences is needed.
On the other hand, ethylene-vinyl alcohol copolymers (hereinafter, may be abbreviated as “EVOH(s)”) are polymeric materials that are superior in barrier properties against gas such as oxygen, oil resistance, antistatic properties, mechanical strength and the like; therefore, resin compositions containing EVOH have been widely used as molding materials of containers, sheets, films and the like.
However, since EVOH has a comparatively active hydroxyl group in its molecule, an oxidation reaction and/or a crosslinking reaction is caused in a molten state at high temperatures even in the interior of an extrusion molding machine in a state being almost free from oxygen, and thus thermal deterioration products may be generated. Particularly, when a continuous operation is carried out for a long period of time, the thermal deterioration products are deposited in the molding machine to generate gels and seeds (i.e., dirt under paint) that account for fish eyes, thereby leading to a disadvantage that the resin composition containing EVOH has insufficient long-run workability.
In order to ameliorate the disadvantages described above, a variety of resin compositions containing EVOH have been developed. For example, a resin composition containing EVOH having improved long-run workability in melt molding by containing a boron compound, sodium acetate and magnesium acetate (see Japanese Unexamined Patent Application, Publication No. H11-60874), a resin composition containing EVOH that enables generation of gels and seeds by melt molding to be suppressed by containing a conjugated polyene compound (see Japanese Unexamined Patent Application, Publication No. H9-71620), a resin composition containing EVOH that is superior in thermal stability and is accompanied by suppressed formation of oxidized gels at high temperatures, due to containing a specific carboxylic acid metal salt and a hindered phenol antioxidant (see Japanese Unexamined Patent Application, Publication No. H4-227744), and the like have been developed.
However, long-run workability of these conventional resin compositions containing EVOH is not considered to be sufficient, and in a case where a metal salt is contained in the composition in a large amount, yellowing of the resulting molded articles such as containers occurs, leading to a disadvantage of being likely to have unfavorable appearances.
In addition, a production method of EVOH has been known in which crotonaldehyde is coexisted in a polymerization step of ethylene and vinyl acetate (see Japanese Unexamined Patent Application, Publication No. 2007-31725). According to this production method, an adhesion of scales in the interior of a polymerization tank can be suppressed by the coexistence of crotonaldehyde during polymerization. As a result, in regard to films of EVOH produced by this production method, generation of fish eyes resulting from the scales detached and contaminated in the polymer can be reportedly decreased.
However, in the production method, crotonaldehyde added in the polymerization is consumed in the polymerization step and a saponification step, and further washed away with water in a washing step and the like; therefore, crotonaldehyde hardly remains in a finally obtained resin composition containing EVOH since the solubility of crotonaldehyde in water is 18.1 g/100 g at 20° C. (see THE MERCK INDEX 14th 2006). Therefore, effects of the unsaturated aldehyde such as crotonaldehyde on an improvement of thermal stability and/or long-run workability in a secondary processing (melt molding, etc.) of the resultant EVOH has been unknown.
To provide a multilayered structure that is superior in the appearance through suppressing generation of gels, fish eyes and the like by improving thermal stability of EVOH has been known (see Japanese Unexamined Patent Application, Publication No. 2001-72823). However, in recent tide of environmental handling such as a reduction in amounts of packaging materials and a reduction in amounts of waste, a reduction in thicknesses of cups, bottles, films and the like has been desired, and there arises needs for additional improvements of the appearances, which have not mattered on conventional packaging materials.